Epoxy resin compositions that contain an epoxy resin and a curing agent for the epoxy resin as essential components exhibit excellent heat resistance and insulating property when formed into a cured product, and are thus widely used in electronic components such as semiconductors and multilayer printed circuit boards.
For electronic component usage, in the technical field of multilayer printed circuit board insulating materials, recently, there have been increases in the speed and frequency of signals in various electronic apparatuses. However, it is becoming difficult to achieve a low dielectric loss tangent while maintaining a sufficiently low dielectric constant with these increases in the speed and frequency of signals.
Therefore, it is desired to provide a thermosetting resin composition capable of producing a cured product which exhibits a sufficiently low dielectric loss tangent while maintaining a sufficiently low dielectric constant for signals at higher speeds and higher frequencies. A known technique for providing materials having such a low dielectric constant and such a low dielectric loss tangent uses an active ester compound as a curing agent for epoxy resin, the active ester compound being produced by aryl-esterifying phenolic hydroxyl groups of phenol novolac resin (see Patent Literature 1 below).
However, although multilayer printed circuit board insulating materials have been anticipated to have extremely high heat resistance due to the tendency for electronic components to have higher frequencies and smaller sizes, the active ester compound produced by aryl-esterifying phenolic hydroxyl groups of phenol novolac resin causes a decrease in the crosslink density of a cured product due to introduction of the aryl ester structure, which results in the insufficient heat resistance of the cured product. Thus, it is difficult to achieve heat resistance while achieving a low dielectric constant and a low dielectric loss tangent.